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Converting a Swimming Pool to Grow Fish

One of the advantages of doing aquaponics is that it gives you the knowledge of how a natural system works.

For Les and Annette Mulder it was only a natural progression to apply their knowledge of fish farming to their 55,000 liter chlorinated swimming pool located in Sydney.

Les switched off the swimming pool pump over a year ago and decided to grow one hundred silver perch the same way a natural system occurs in nature. This meant the fish would have to survive on their own.

Could such a thing work in a suburban sized swimming pool?

The decision to stop putting chlorine into your pool and allow nature to take over can take a bit of courage – especially if your friends think you are mad.

“Our friends think we are green crazy weirdo nuts!” said Les.

Annette laughed, “The pool got darker and murkier and started looking like something from the black lagoon!”

“It really was black and disgusting as the chlorine came out but it was still too toxic for anything to live in there other than some algae and things like that.” said Les.

“Eventually some Mosquito larvae began to turn up in the water and that meant we could throw some Pacific Blue-Eyes (a small variety of fish) in there.”

For filtration Les arranged three bathtubs in a row and filled them with gravel. A small 60 Watt pond pump will be used to recirculate the water through the bathtubs and that water would return filtered clean back to the pool. The growbeds were stocked with aquatic plants such as Taro, Papyrus, Louisiana Swamp Iris, Chinese Water Chestnuts and Mints. Les and Annette say the couple wanted to choose plants that were very good at filtering water.

“The aim is to try and get a system that was in balance faster than the aquaponics system.” said Les.

This 55,000 Litre Swimming Pool is converted to a small fish farm using very little energy

“We want to get to the point where we got with our other aquaponics system, where it becomes self cleaning and self regulating…” says Les. Planting aquatic plants was a way to fast track that conversion.
Les has also placed a large children’s clam shell swimming pool underwater at the right height that is also filled with sand that grows water chestnuts.

All these additional aquatic plants have had an unexpected benefit.

Various microscopic animals eventually moved into this swimming pool system and the fish are thriving living off small insects, mosquito larvae, algae and fresh shoots from plant roots.

The fish were living in a natural system. We were flabbergasted when Les said he does not feed the fish at all. No fish pellets from depleted reserves of ocean catch are needed. According to Les the fish at five months of age were doing extremely well finding their own food. He believes they are growing faster than if they were fed on commercial fish pellets. The system is looking after itself.

The other thing the couple noticed were the re-emergence of rare frogs that came to live in their pool system.

“We even got a species that we never had around her before.” said Les. The emerald spotted tree frog began croaking from the gravel bathtubs.

We were filming at this pool with Permaculture teacher Geoff Lawton who agreed that most people spend a lot of money actively trying to stop algae from appearing in their pools, yet here was a system where algae was encouraged to grow and help feed the fish. There is some evidence to support the theory that some algae are also very high in Omega-3 fatty acids a health benefit to people that will eventually eat the fish. The added benefit as Les also discovered is the drop in electricty costs from switching off his main pool pump. Rather than 1 kilowatt of energy spent running a conventional chlorinated pool per hour, he had reduced his costs to a fraction. Because of the large surface area of the pool, little aeration was also needed. A small trickle pvc pipe returns a steady stream of water back to the pool, which provides the only air back into the system. Before you run off and build your own system, be mindful that this system requires a large body of water (50,000 litres) to become effective.

One hundred Silver Perch in such a system are at a very low stocking density when compared to conventional aquaponics systems. But the advantages are great. Mechanical failure of pumps means the fish will survive if you forget to turn the power on. Fish will find their own food. The nutrients that the fish provide will also be on the low side so three bath tubs of filtration may be all the food you can grow unless you decide to add more fish. But for a leisurely approach to taking it slow, easy and letting nature do most of the work – getting 30 to 50 kilos of fish at the end of the year seems like a real bonus for very little inputs.

This system will be featured in The Urban Permaculture DVD with Geoff Lawton later in the year.

7 Comments

Thanks for moving forward and taking an idea to its next level.Im living here in Florida US and would like to learn more.Im attempting to do the same thing.with a pool oof 10000 gals.by building an outside of pool gravel filter.Email me and I would like to talk more Paul

I have a question I want to buy an outdoor pool of 8127 liters to make my aquaponics, what range of fish would you recommend and media beds for the planting. One last question and the pump.thanks in advance.

Hi Luis, Check out the Stocking Densities post here: http://www.ecofilms.com.au/2010/10/25/stocking-densities-in-aquaponics/
We run around 30 – 50 fish per 1000 litre fish tank but we have lots of oxygen and a 66Watt 4500 litre per hour pond pump running 24/7 which turns the water over very quickly. Stock densities are pretty much based on adequate fish mass per given volume of water and the ability to run your tanks to it’s level of filtration. A very important rule. No good to have lots of fish if you cant get rid of the waste they produce. You stock more fish when they are small and eat them as they grow and keep fewer as they mature in the same volume of water. It keeps everything in balance and easy to remember. Its explained in the Aquaponics Made Easy DVD a little more clearly. In an 8,000 litre capacity tank you could have it only half full and run fewer grow beds. A larger volume of water will give you more stability and keep the fish happier should the pump fail. Less is more when it comes to fish. Most people in Aquaponics use less than 2,000 litre fish tank and similar sized capacity of grow beds to supply all the food they need for their family. A big swimming pool like the one featured above is very lightly stocked when you consider the fish numbers to be around only 100. Imagine if there were 10,000 fish in there. You would need massive pumps and heavy filtration to get rid of the waste and keep the system in balance. Sometimes less is more…

I float my vegetables on top of the water, the fish, in this case are redclaw crayfish, live underneath. the float mats are purpose built with snap in cups. Tha water is 300mm deep, the pool is 5mt x 9mt square, 3000 plants at 70% surface coverage.

Hi there, we were thinking of converting our pool to an aquaponic system and it appears to work in the above example – however in Murray’s videos he says converting swimming pools doesn’t work in a normal suburban situation. I am confuzzled now!!

He was only referring to the volume of nutrient in the water being equal to the volume of grow beds. If you reduced the volume of water in your swimming pool then it would work fine because the nutrient would be less diluted. Try and picture the volume of the water having sufficient nutrient to feed the plants.